I. Field
The present disclosure relates generally to communication, and more specifically to pilot transmission in a communication system.
II. Background
In a communication system, a base station processes traffic data to generate one or more modulated signals and then transmits the modulated signal(s) on a forward link (FL) to one or more terminals. The forward link (or downlink) refers to the communication link from the base station to the terminals, and the reverse link (or uplink) refers to the communication link from the terminals to the base station. The base station may serve many terminals and may select a subset of these terminals for data transmission on the forward link at any given moment.
The base station can typically improve the performance of the FL data transmission by employing advanced scheduling and/or transmission techniques. For example, the base station may schedule terminals in a manner to account for frequency selective fading (or a non-flat frequency response) observed by the terminals. As another example, the base station may perform beamforming to steer the FL data transmission toward the scheduled terminals. In order to employ the advanced scheduling and/or transmission techniques, the base station typically needs to have reasonably accurate estimates of the forward link channel responses between the base station and the terminals.
In a frequency division duxplexed (FDD) system, the forward and reverse links are allocated separate frequency bands. Consequently, the forward link channel response may not correlate well with the reverse link channel response. In this case, the terminals would need to estimate their forward link channel responses and send back the forward link channel estimates to the base station. The amount of signaling needed to send back the forward link channel estimates is typically prohibitive and thus limits or prevents the use of the advanced techniques for the FDD system.
In a time division duxplexed (TDD) system, the forward and reverse links share the same frequency band. The forward link is allocated a portion of the time and the reverse link is allocated the remaining portion of the time. In the TDD system, the forward link channel response may be highly correlated with, and may even be assumed to be reciprocal of, the reverse link channel response. For a reciprocal channel, the base station can estimate the reverse link channel response for a terminal based on a pilot transmitted by that terminal and can then estimate the forward link channel response for the terminal based on the reverse link channel estimate. This can simplify channel estimation for the forward link.
As noted above, the base station may serve many terminals. Requiring pilot transmission from all terminals all of the time may lead to extremely inefficient utilization of the system resources. This inefficiency may manifest itself as higher interference to other base stations and a larger overhead for pilots on the reverse link.
There is therefore a need in the art for techniques to more efficiently transmit pilots in a communication system.